Hitherto known syntheses of compounds of the Formula I are based on the parent compound (R.sup.1 =R.sup.2 =H, X=OH), which can be obtained in optically active form by classical resolution of the racemate E. Felder et al., Helv. Chim. Acta, (1960), 43, 888-896!. First the ring nitrogen atoms are protected and then the free carboxyl group is converted to the amide. At best the process is suitable for the laboratory scale and it requires the use of expensive reagents.